In the prior art, corrugated sealing membranes intended to form an internal coating for liquefied natural gas storage tanks are known. The sealing membrane is made up of a plurality of metal plates having perpendicular series of corrugations allowing it to deform under the effect of the thermal and mechanical stresses generated by the fluid stored in the tank.
Such a corrugated sealing membrane is described for example in document KR100766309. The corrugated membrane comprises a first series of parallel corrugations, referred to as tall corrugations, extending in a first direction and a second series of parallel corrugations, referred to as low corrugations, extending in a second direction perpendicular to the first.
In order to create such a sealing membrane the tall corrugations are formed first of all and then the low corrugations and the node zones between the low and tall corrugations are then formed.
The aforementioned document KR100766309 provides for the tall corrugations to be created by means of a bending device comprising a lower frame and an upper frame mounted with the ability to move vertically with respect to the lower frame between a rest position and a bending position. The lower frame supports a die made up of two die elements each having a half-cavity, these being able to move between a spaced-apart position and a close-together position one against the other in which the half-cavities of the two die elements together define a cavity corresponding to the shape of the corrugation that is to be formed. Moreover, the upper frame carries a punch intended to engage inside the cavity so as to form the corrugation, when the upper frame is in its bending position, and two clamps extending one on each side of the punch and collaborating respectively with each of the die elements when the upper frame moves toward its bending position so as to hold the metal sheet against the die elements so that the metal sheet remains in position as it is being bent.
During a bending operation, the two die elements move closer together at the same time as the punch moves toward its bending position. Thus, such a bending device makes it possible to ensure that the bending operation does not alter the thickness of the metal sheet, notably at the corrugation. This is because it is absolutely essential for the metal sheet after bending to have a thickness that is constant so as not to impair its mechanical properties.
However, such a bending device allows only one corrugation to be made at a time. Thus, in order to create a plurality of corrugations in the metal sheet, one stroke of the press needs to be performed for each corrugation and the metal sheet needs to be moved between each stroke of the press. Thus, the operations for creating a plurality of corrugations in one and the same metal sheet are lengthy and complex to implement.